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Pii: s0925-3467(99)00119-6

Optical Materials 14 (2000) 101±107 Optical properties of lissamine functionalized Nd3‡ complexes in polymer waveguides and solution L.H. Sloo€ a,*, A. Polman a, S.I. Klink b, G.A. Hebbink b, L. Grave b, F.C.J.M. van Veggel b, D.N. Reinhoudt b, J.W. Hofstraat c a FOM Institute for Atomic and Molecular Physics, Kruislaan 407, 1098 SJ Amsterdam, The Netherlands b Supramolecular Chemistry and Technology, University of Twente, P.O. Box 217, 7500 AE Enschede, The Netherlands c Philips Research, Department of Polymers and Organic Chemistry, Prof. Holstlaan 4, 5656 AA Eindhoven, The Netherlands Received 3 May 1999; accepted 5 November 1999 Lissamine functionalized terphenyl-based Nd complexes are synthesized, and incorporated in deuterated dimethyl- sulfoxide solutions and partially ¯uorinated planar polymer waveguides. Optical excitation of the lissamine sensitizer around 500 nm, followed by intramolecular energy transfer to the Nd3‡ ion, causes near-infrared photoluminescence (890, 1060, 1340 nm) due to intra-4f transitions in the Nd3‡ ion. The intramolecular energy transfer rate is larger than 107 sÿ1. Due to the large absorption cross-section of the sensitizer (>10ÿ17 cm2 around 500 nm), the Nd3‡ is excited 104 times more eciently than in a pure complex, without sensitizer. The Nd3‡ luminescence lifetime is relatively short, both in solution (2.2 ls) and in a polymer host (0.8 ls), which is attributed to coupling to vibrational states of nearby C±H and O±H groups. Spincoated ¯uorinated polymer planar waveguides, doped with these sensitized organic Nd complexes show excellent waveguide properties. Upon continued illumination, photodegradation is observed in the doped polymer ®lms. Ó 2000 Elsevier Science B.V. All rights reserved.
the parity forbidden transitions slightly allowed.
The lifetimes of these transitions are therefore Trivalent rare earth ions are well known for relatively long.
their special optical properties [1]. The 4f-shell of These properties make the rare earth ions useful these ions is not completely ®lled and shielded for applications in integrated optics. The rare from the surroundings by ®lled 5s and 5p shells.
earths erbium (Er) and neodymium (Nd) are This shielding minimizes the e€ect of the crystal commonly used in optical ®ber ampli®ers [2±5] due ®eld of the host material on the energy levels of the to their intra-4f transitions at 1550 nm (Er, 4f-shell and as a result the absorption and emission 4I13=2 ! 4I15=2) and 1340 nm (Nd, 4I3=2 ! 4I11=2), bands remain rather sharp. The small in¯uence of two standard telecommunication wavelengths. A the crystal ®eld induces mixing of wave functions lot of research has focused on rare earth-doped with opposite parity within the 4f-shell, making solid-state planar optical ampli®ers for integrated optics applications, and working planar ampli®ers have been reported for rare earth ions in silica, * Corresponding author.
Al2O3, phosphate glasses, and LiNbO3 [6,7]. The 0925-3467/00/$ - see front matter Ó 2000 Elsevier Science B.V. All rights reserved.
L.H. Sloo€ et al. / Optical Materials 14 (2000) 101±107 growing importance of polymer thin ®lms in inte- grated optics technology makes it interesting to study rare earth doped polymer waveguides, and see if polymer optical ampli®ers can be made [8,9].
These polymer waveguide ampli®ers could then be integrated with existing polymer devices such as splitters, switches, and multiplexers [10].
Rare earth ions cannot be dissolved directly into a polymer ®lm. Therefore, the ions have to be encapsulated by an organic ligand to form a complex, which can be dissolved in the polymer matrix. Previously, we have shown that optically Fig. 1. Schematic energy level diagram of the lissamine±Nd3‡ active Er-doped polydentate cage complexes can complex. The arrows indicate the excitation mechanisms of the be synthesized, and show room temperature pho- Nd3‡ ion: either directly into the 4G7=2 level by pumping at 515 nm, or through the lissamine sensitizer (S toluminescence at 1.535 lm when optically excited 0 ® S1 transition fol- lowed by intersystem crossing and energy transfer).
either directly into an Er level, or indirectly via the cage which also acts as a chromophore [11]. We found that the luminescence lifetime of these tizer, the 515 nm light is mainly absorbed by the complexes is rather short (0.8 ls), which was at- high absorbing lissamine, which becomes excited tributed to energy transfer of the excited state of into the singlet state (S1). This is followed by the Er3‡ ion to ±OH and ±CH vibrational states of intersystem crossing to the triplet state (T, the complex or of the solvent molecules [12,13].
ET ˆ 14600 cmÿ1) (see Fig. 1). From the triplet Deuteration of ±CH and ±OH bonds reduces some state energy transfer to the Nd3‡ ion can occur, of these quenching paths [14].
which results in excitation of the Nd3‡ ion into the Obviously these non-radiative quenching pro- 4S3=2 and 4F9=2 levels. After relaxation to the 4F3=2 cesses are a disadvantage of the use of organic cage level the 890, 1060 and 1340 nm luminescence can complexes. On the other hand, an advantage is be observed. The luminescence intensities, life- that highly absorbing antenna chromophores can times, excitation mechanisms, waveguide proper- be incorporated in the organic complex. Once this ties, and photostability of these complexes will be chromophore is excited it can transfer its excita- tion energy to the rare earth ion. If the energy transfer from chromophore to rare earth ion is ecient, this process strongly enhances the exci- tation eciency of the rare earth ion.
In this paper, we will report the optical prop- Terphenyl-based Nd3‡ complexes were synthe- erties of terphenyl-based Nd3‡ complexes with and sized [15] using the procedure described in Ref.
without a highly absorbing lissamine antenna [15]. Some complexes were functionalized with chromophore. Complexes were dissolved either in lissamine, a Rhodamine-B derivative [16]. Fig. 2 hexadeutero-dimethylsulfoxide (DMSO-d6) solu- shows a schematic picture of the structure of the tions or in partially ¯uorinated polycarbonate terphenyl-based Nd3‡ complexes (a) with two planar waveguides. In complexes without lissa- benzoyl side-groups (Bz.Nd) and (b) with one mine, excitation of the Nd3‡ ion at a wavelength of benzoyl side-group and a lissamine sensitizer 515 nm leads to population of the 4G7=2 level, from (Ls.Nd). Both complexes have a cage-like con®g- where it decays to the 4F3=2 level (see Fig. 1). De- uration, encapsulating the Nd3‡ ion. The com- cay from this level leads to the characteristic Nd3‡ plexes were dissolved in DMSO-d6 to a luminescence at 890, 1060 and 1340 nm due to concentration of 10ÿ2 M for Bz.Nd and 10ÿ6 M for transitions to the 4I9=2, 4I11=2 and 4I13=2 levels, re- Ls.Nd, or dissolved in partially ¯uorinated poly- spectively. In complexes with the lissamine sensi- carbonate [17] waveguides at a concentration of

L.H. Sloo€ et al. / Optical Materials 14 (2000) 101±107 Fig. 2. Schematic picture of (a) benzoyl±Nd3‡ complex (Bz.Nd) and (b) lissamine functionalized Nd3‡ complex (Ls.Nd).
3 wt% (complex). The polycarbonate waveguides the entrance facet of the waveguide. The lumines- were made by spincoating a cyclohexylacetate so- cence was collected at the output facet, using a lution of polycarbonate and complex onto a Si multi-mode optical ®ber. The ®ber was led to an substrate covered with a 3 lm thick thermally objective in front of the monochromator. The grown SiO2 layer. The spincoating was performed spectral resolution ranged from 0.2 to 6 nm.
for 30 s at a spinrate of 3000 sÿ1 and followed by Photoluminescence decay curves were measured thermal annealing at 190°C (in vacuum) for 1 h.
using a photomultiplier tube and a photon The thickness of the polymer layer was 3.55 lm.
counting system. The time resolution of the system The real and imaginary parts of the refractive was about 100 ns.
index of the polymer waveguide were measured using a variable angle spectroscopic ellipsometer.
The optical losses were measured using the sliding 3. Results and discussion prism method [18]. Diiodomethane was used as an index matching liquid for optimum output cou- Fig. 3 shows the PL intensities at 1060 nm for a pling. A white light source as well as lasers oper- 10ÿ2 M solution of Bz.Nd and a 10ÿ6 M solution ating at 633, 838, 1305 and 1565 nm were used.
of Ls.Nd (lissamine sensitized), both in DMSO-d6, Photoluminescence (PL) measurements were at di€erent excitation wavelengths as available performed using various lines of an Ar-ion laser as from the Ar-ion laser. The absorption spectrum an excitation source. The complexes in DMSO-d6 measured for the same solutions is also included in solution were analyzed in square quartz cells. The the ®gure. The pump power for excitation was 60 power on the cell was 60 mW, at a spot diameter of mW for all excitation wavelengths. The excitation 1 mm. The beam was modulated using an acousto- spectrum for the complex without sensitizer optic modulator, operating at di€erent frequen- (Bz.Nd) shows some structure, which is roughly cies. The emitted luminescence was focused into a similar to that found in the absorption measure- monochromator and detected with a photomulti- ment, and is consistent with the absorption bands plier tube or a liquid-nitrogen-cooled Ge detector.
of the Nd3‡ ion around 475 and 513 nm. The ex- All spectra were corrected for the detector re- citation spectrum for the complex with sensitizer sponse. Absorption measurements were performed (Ls.Nd) shows a completely di€erent behavior: the using a spectrophotometer. Measurements on 1060 nm emission intensity increases strongly with waveguide ®lms were performed by pumping the excitation wavelength, again very similar to what waveguide (total length about 25 mm) with a is found for the absorption spectrum. Given the rectangular spot …5  15 mm† from the top near fact that the lissamine complex shows a broad L.H. Sloo€ et al. / Optical Materials 14 (2000) 101±107 temperature PL of Nd3‡ at 890, 1060, and 1340 nm. Although the concentration of Ls.Nd is 104 times lower than the concentration of Bz.Nd, the PL intensity is two times higher. Optical absorp- tion measurements at 515 nm for both solutions show an almost equal absorption: 0.033 cmÿ1 for 10ÿ2 M Bz.Nd and 0.031 cmÿ1 for 10ÿ6 M Ls.Nd.
The fact that the sensitized complex shows higher luminescence than the complex without a sensi- tizer, even though the measured absorption was the same, indicates that the internal energy trans- fer eciency within the sensitized complex is quite Fig. 3. Photoluminescence at 1060 nm as a function of excita- high. The factor 2 di€erence can be due to the fact, tion wavelength for Bz.Nd (10ÿ2 M, squares) and Ls.Nd (10ÿ6 M, circles) in DMSO-d6 solutions. The absorption spectra of that upon direct excitation into the higher lying Bz.Nd and Ls.Nd are also shown (drawn lines). The absorption state of the Nd3‡ ion, the Nd3‡ ion can also decay data for the Bz.Nd solution are multiplied by a factor of 2000.
radiatively to the ground level (indeed, 524 nm luminescence has been observed, resulting from the transition from the 2K absorption band around 580 nm, this clearly in- 13=2 ! 4I9=2 transition), leading to a lower quantum yield for the near-in- dicates that the excitation of Nd3‡ around 500 nm frared transitions in the case of direct optical ex- takes place via the sensitizer. The absorption of the citation of the Nd3‡ at 515 nm. The measured lissamine occurs at the xanthene unit (i.e. the gray luminescence lifetime (not shown) at 1060 nm for part in the structure for Ls.Nd in Fig. 2(b)). Note the Bz.Nd complex in DMSO-d6 is 2.5 ls, and for that the measured absorption cross-section is in the sensitized Ls.Nd complex it is 2.2 ls. The lu- the 10ÿ17 cm2 range, which is four orders of mag- minescence lifetime of Nd3‡ in inorganic materials nitude above the typical Nd3‡ intra-4f transition can be as high as 250 ls [19]. The low quantum yield in the organic complexes is attributed to Fig. 4 shows the room-temperature PL spectra quenching of the Nd3‡ excited state by coupling to for 10ÿ2 M Bz.Nd and 10ÿ6 M Ls.Nd in DMSO- overtones of nearby C±H and O±H vibrational d6, recorded using excitation at 515 nm at a pump power of 60 mW. The complexes show room- Fig. 5(a) shows the measured (dashed line) and simulated (solid line) ellipsometry parameter D as a function of wavelength for a partially ¯uorinated polycarbonate planar polymer waveguide doped with 3 wt%. Ls.Nd. The interference structure is caused by re¯ections at the air/polymer, polymer/ SiO2, and SiO2/Si interface. A clear dip in D is observed around 580 nm, which is caused by the high absorption of the lissamine. The simulated data are based on a Lorentz oscillator model and correspond well with the measured data. From the simulation parameters, the real (n) and imaginary (k) part of the refractive index of the Nd-doped polymer waveguide layer can be calculated and they are shown in Fig. 5(b). Also shown is the measured refractive index of an undoped reference Fig. 4. Photoluminescence spectra of Ls.Nd (10ÿ6 M) and Bz.Nd (10ÿ2 M) in DMSO-d6 solutions. The excitation wave- polymer. Outside the resonance region, the re- length is 515 nm at a pump power of 60 mW.
fractive index of the Nd-doped waveguide layer is L.H. Sloo€ et al. / Optical Materials 14 (2000) 101±107 Fig. 6. Optical loss spectrum of an undoped partially ¯uori- nated polycarbonate waveguide, also shown is the measurement Fig. 5. (a) Measured and calculated ellipsometric parameter D as a function of wavelength for a 3 wt% Ls.Nd-doped partially these polycarbonate waveguides are ideally suited ¯uorinated polycarbonate waveguide. (b) Real (n) and imagi- for planar waveguide applications.
nary (k) part of the refractive index derived from the simulation Fig. 7 shows the PL intensity of a 3 wt%. Ls.Nd data indicated by the solid line in (a). The real index for an doped ¯uorinated polycarbonate waveguide after undoped reference waveguide is shown for reference (dashed line in (b)).
excitation at a wavelength of 515 nm. The dashed line shows the luminescence measured after exci- tation and collection of the light from the top of very similar to that of the undoped layer, indi- the sample …P ˆ 40 mW†. The 890 and 1060 nm cating that the spincoating technique leads to Nd- luminescence of the Nd3‡ ion are clearly seen.
doped waveguide layers with similar density as pure waveguide layers. The maximum value of k is 0.027 at 580 nm which corresponds to an absorp- tion cross-section of 4:5  10ÿ16 cm2, which is roughly 2±3 times higher than the literature value for Rhodamine-B [14]. It is clearly seen that the high absorption of the lissamine causes a change in the real part of the refractive index around 580 nm as described by Kramers±Kronig theory.
Optical loss measurements were performed on an undoped partially ¯uorinated polycarbonate waveguide using the prism coupling technique.
The result is shown in Fig. 6. The peak around 1650 nm is due to overtone absorption by C±H bonds. The band around 1400 nm is attributed to Fig. 7. Photoluminescence spectra of a 3 wt% Ls.Nd-doped absorption by C±H bonds and O±H bonds in the polymer waveguide. The dashed line indicates the spectrum polymer. The two peaks around 1150 nm arise collected from the top of the waveguide after excitation at 515 from second overtone absorption by aromatic and nm at a power of 40 mW. The solid line indicates the spectrum aliphatic C±H bonds. The background loss at the after excitation at 515 nm at a power of 140 mW and collected from the output face of the waveguide using a multimode op- Nd3‡ emission wavelengths is <0.05 dB/cm at 1060 tical ®ber. The inset shows the time dependence of the 1060 nm nm and 0.08 dB/cm at 1305 nm. This indicates that luminescence upon pulsed excitation.
L.H. Sloo€ et al. / Optical Materials 14 (2000) 101±107 Spectra taken in the near-infrared region (not A. According to Dexter [21], this means shown) also show the 1340 nm luminescence. The that 3% of the maximum possible energy transfer emission observed in the lower wavelength region rate is reached. This could be improved by re- is due to luminescence of the lissamine, which has ducing the distance between the lissamine and the a peak emission wavelength (S1 ! S0 transition) Nd3‡ ion by changing the con®guration of at- at 580 nm. The small peaks at the shoulder of the tachment of the lissamine. Another possibility to lissamine luminescence are attributed to an inter- increase the energy transfer rate is to improve the ference e€ect between the waveguide layers. The spectral overlap between the sensitizer and the solid line in Fig. 7 is the spectrum observed for Nd3‡ ion [22]. In the present case, the energy of excitation from the top and collecting from the the triplet state of lissamine matches with the 14 600 output facet of the waveguide …P ˆ 140 mW; cmÿ1 band of Nd3‡ which shows weak absorption.
spot 5  15 mm2†. Again the 890 and 1060 nm If a sensitizer could be used that matches the luminescence are observed, but here the lumines- strong 13 600 cmÿ1 absorption band, the energy cence of the lissamine is strongly decreased, which transfer rate could be increased.
is attributed to re-absorption by the lissamine it- Finally, photo-degradation measurements of self. This indicates that self-absorption should be the 580 nm lissamine luminescence were performed taken into account when designing waveguide on polymer waveguides doped with 3 wt% (com- plex) and are shown in Fig. 8(a) for normal inci- The inset of Fig. 7 shows the time dependence dence irradiation at three di€erent pump powers.
of the 1060 nm PL signal after switching the 515 The luminescence signal was detected normal to nm pump on and o€. An exponential ®t through the waveguide. For a pump power of 4 mW the the decay part results in a PL lifetime of 0.8 ls.
luminescence intensity slowly rises to a maximum This is signi®cantly lower than the decay measured and then decreases again. At higher pump powers for complexes in DMSO-d6 solution (2.2 ls), the same e€ect is observed, but both the increase which indicates that quenching by C±H and O±H and the decrease processes occur faster. The pho- groups in the polymer matrix also contributes to to-degradation process may be the result of oxygen the radiationless deactivation of the Nd3‡.
induced quenching. Another possibility is that The energy transfer from the lissamine to the upon photo-excitation, radicals are formed which Nd3‡ ion occurs via the triplet state of the lissa- react with the sensitizer [23]. Further research is mine. However, the triplet state can also be quenched by e.g. O2, that is, always present in solution or in a polymer ®lm. In order to investi- gate this, we measured the Nd3‡ luminescence in- tensity of Ls.Nd in DMSO-d6 solutions before and after degassing (not shown). The Nd3‡ PL inten- sities were similar, indicating that quenching by O2 does not play a role in the DMSO-d6 solutions. As it is known that the quenching rate by O2 is about 107 sÿ1, it can be estimated that the intramolecular energy transfer rate to the Nd3‡ ion has to be >107 sÿ1. This transfer rate is of the same order of magnitude as for triphenylene sensitized Eu3‡ in acetonitrile solutions [20]. A space ®lling model of Ls.Nd, in which all atoms are represented by their Fig. 8. (a) 580 nm lissamine photoluminescence of a 3 wt% van der Waals radius, shows that the distance Ls.Nd-doped waveguide at pump powers of 4, 20 and 80 mW.
between lissamine and the Nd3‡ ion is 7±8  (b) Similar measurements on a 3 and 1 wt% Ls.Nd-doped waveguide at a pump power of 10 mW. As can be seen, the 1 whereas the e€ective average Bohr radius for the wt% luminescence does not show the initial increase in the in- excited lissamine and the unexcited Nd3‡ ion is L.H. Sloo€ et al. / Optical Materials 14 (2000) 101±107 going on to determine the exact nature of the coating the polymer waveguides and Hans Lamers (Akzo Nobel) for performing the prism coupling Room-temperature photoluminescence of Nd3‡ complexes in DMSO-d6 solution and partially [1] S. Hufner, Optical Spectra of Transparent Rare-Earth ¯uorinated polycarbonate waveguides has been Compounds, Academic Press, New York, 1978.
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[11] L.H. Sloo€, A. Polman, M.P. Oude Wolbers, F.C.J.M. van tributed to coupling to O±H and C±H vibrations Veggel, D.N. Reinhoudt, J.W. Hofstraat, J. Appl. Phys. 83 of the complex and the host material. Good quality optical waveguides can be spincoated with [12] G. Stein, E. Wurzberg, J. Chem. Phys. 62 (1975) 208.
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observed that might be due to the presence of [15] S.I. Klink, G.A. Hebbink, L. Grave, F.C.J.M. van Veggel, oxygen in the polymer ®lm or due to radicals, D.N. Reinhoudt, J.W. Hofstraat, L.H. Sloo€, A. Polman, which are formed upon photo-excitation.
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and has been ®nancially supported by the council [20] E. van der Tol, Ph.D. Thesis, University of Amsterdam, for Chemical Sciences of the Netherlands Organi- Amsterdam, 1998.
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the IOP Electro-optics Program and the SCOOP [22] M.H.V. Werts, J.W. Hofstraat, F.A.J. Geurts, J.W.
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program of the European Union. Benno Hams [23] R.P. Wayne, Principles and Applications of Photochemis- (Akzo Nobel) is greatly acknowledged for spin- try, Oxford University Press, Oxford, 1988.

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